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Do helicopters have to pay landing fee?
Is there a landing fee for helicopters?
I ask because since they usually don't use the runway hence doesn't cause maintenance costs for the airport I would imagine they have at least a different fee to pay if they have to pay?
Is that how it works?

They may not use the runway, but they still use airport infrastructure, handling, ATC and so on

In the UK landing fees are pretty much ubiquitous at all airfields, from large commercial airfields down to small GA airfields. Typically the fees for helicopters are at least as much as fixed wing aircraft and often times higher.
The most popular GA airfield in the UK publishes their charges and as you can see from that page they charge as much for a small helicopter (such as a R22) as for a light 4 seater (PA28).
Another airfield I know very well, which is very close to London and often used for helicopter traffic to central London, the Grand Prix and other such events visited by those fortunate enough to use helicopter transport also publish their charges and for that airfield the charges are much higher for helicopter landing fees ranging from £25 up to £350.
So yes, helicopters absolutely pay landing fees where it is the standard practice to do so. They may not use the runways, but they often have a dedicated heli-apron, dedicated ground crew (helicopters cant maneuver themselves into hangars/parking, and often need specialist equipment and handling), dedicated fuel bowsers (again, often unable to taxy themselves to fuel). All of this costs money and time, and in the UK at least, is funded in part by landing fees.

I won't attempt to answer for whatever country you are in but I think we can assume that my situation is somewhat similar to yours too.
My nearest airport is Mumbai Intl. and according to this Airport Charges file I found, that there IS a charge for all types of aircraft. Usually, helicopters land at the smaller airport that is right next to the international one but the charges are similar I believe.
The documents I've seen show that they don't look at your runway usage, they just see whether you're on the airport grounds, or not.
For Mumbai Intl. in the FY2013-14 file it's mentioned that

A minimum fee of Rs. 16,341 and Rs. 21,788 shall be charged per single
domestic and international landing respectively for all types of aircraft/
helicopter flights, including but not limited to domestic landing, international
landing and general aviation landing.

There's more charges based on your aircraft weight too.
So that's there and there's more charges that're there in that file. It's very outdated but the question is more yes or no so I think this file is perfectly fine.
So for the question, do they have a different fee; mostly no, but by weight, passenger capacity, and other factors, their fees will be lowered significantly or at least slightly
Currency Conversions:

circa 2013, ₹1 = $0.018

current rate as of writing this, ₹1 = $0.012

ok, I was thinking a Cessna 172 and a Robinson R44 both have similar weight and passenger capacity, but the helicopter doesn't use the runway still has to pay the same.... just thinking....

@Gabe yes, but landing fees encompasses a lot of things other than simply runway utilization.

yeah, that's a fair way to put it.

In the US, in most public airports, there is no landing fee. For General Aviation, airports are financed via fuel surcharges, which are can be anything between one dollar to five dollars per gallon, from my observations. In the US, helicopters can land on taxiways or have dedicated landing spots. In the US, you can also land a helicopter anywhere, where the landowner allows it. One guy in my neighborhood lands his on an empty lot, in a suburban neighborhood with otherwise high density.


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How does the pressurization system（B737）work if airplane climbs again after descending from the original cruising level？
Does the pressurization system（B737ng） still remain in descent mode？

Assuming AUTO mode operation, descent mode would activate when the aircraft descends 0.25 psi below the selected FLT ALT (≈2000 ft at 37,000ft). The cabin begins a proportional decent to slightly below the selected LAND ALT.
If a climb was then initiated, ambient static pressure would start to decrease, altitude would increase and thrust levers would advance. Ambient static pressure and altitude changes are detected by the air data inertial reference units (ADIRUs) and thrust lever position are detected by both stall management computers (SMYDs). The cabin pressure controller receives inputs from the ADIRUs and SMYDs and would act to maintain a proportional pressure differential between airplane altitude and cabin altitude. It would therefore no longer be in descent mode.

If the FLT ALT window wasn’t altered, and the aircraft climbed again to the previous cruising level, the cruise mode would activate again when the airplane climbs within 0.25 psi of the FLT ALT (≈2000 ft at 37,000ft). In cruise mode the cabin pressure controller seeks to maintain the lowest possible cabin altitude  based on differential pressure limits.

If the FLT ALT window wasn’t altered, and the aircraft climbed again above the previous cruising level, the cabin pressure controller would act to maintain a proportional pressure differential between airplane altitude and cabin altitude. This may result in a higher cabin altitude than if the FLT ALT window was adjusted to the actual new cruising altitude. The cabin pressure controller is not in cruise mode. Furthermore, as the system is not anticipating a new FLT ALT it may take a short amount of time to “settle” at a stable cabin altitude when the new cruising altitude has been reached. This delay happens because the cabin pressure controller needs to adjust the outflow valve position to balance the pressure differential and achieve a new stable cabin environment.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

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Has any TFR been issued due to “abnormally high pressure”?
TFR is supposed to be issued if the barometric pressure is more than 31.00”Hg as it is greater than altimeter adjustment range. Has this type of TFR been issued before within the U.S.?

I mean TFR is supposed to be issued if the pressure is more than 31.00"Hg to prohibit flight operation. And I was wondering if that would occur within the U.S. or not.

I see you are at KDAB where the pressure got up to the upper 30.40s at some point this past week.

There has been an episode of extreme cold in Alaska in February 1989. The pressure exceeded 31 inHg. According to the LA Times:

Barometric pressure, which had reached a North American record of 31.84 inches on Tuesday, was still so high in Alaska that the Federal Aviation Administration banned night and instrument flights because altimeters could not be accurately calibrated to give altitude readings.

That's not the pressure itself which is a problem, but the QNH that must be entered in the altimeter setting window, calculated by extrapolating this pressure to sea level (0.1 inHg added for each 100ft of altitude of the measurement location).
During this episode a C-130 crashed  at Fort Wainwright's Ladd Army Air Field, with a pressure of 30.79 inHg (I assume it's the QNH) just below the 31 inHg limit for altimeter setting. The cause was more the temperature (-52°C) and the icing fog.
QNH history at Fairbanks airport:

Source.
As confirmed by @Chris, at this time 14 CFR 91.114 didn't exist, so the TFR wasn't based on it. However the principle was written into the law in 1994, first as 91.92, then moved to 91.114 to be grouped with other TFR articles. The law justification included this 1989 event as well as the increasing possibility of such events in the next 10 years.
High pressure and low temperature are typical of Siberian climate, though the current world record for high pressure is 32.25 inHg (1089.1 hPa) at Tosontsengel in Mongolia on 29 December 2004.

"Some sites state this event actually caused the restriction to be added to the FAA regulations." These sites are correct. 91.114 was added in 1994 (initially as 91.92) and the FAA specifically cites the 1989 event as justification for the new regulation. (See 59 FR 17450).

@Chris: Thanks, that's a very useful information, added in my answer.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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How does a paraplegic jump off a slide in an emergency?
On a typical airliner, or any other airplane, what is the emergency procedure for a paraplegic (or other disabled people) to be able to get out of the airplane?
Obviously the flight attendants will need to assist anyone who is mobility impaired to the exits, but I'm looking for a specific example of the wording used to establish how this will be performed, and in what sequence.
Consider for example:

Someone (presumably a flight attendant) will need to exit first to be in a position to catch them and help them out of the way.
Moving the person from their seat to the exit may require several attendants depending on the person's weight, and doing so will block the aisle and impede able bodied passengers.

Does anybody have an example of specific airline procedures describing how such an evacuation is to be performed?

What are you looking for besides "the flight attendants help"?

He doesn't go first, because there would be nobody to catch him. And he doesn't go last, because there would be nobody to throw him.

Seriously though, there must be a procedure for this, anyone with cabin crew training please chime in!

@Jpe61 that's what I'm saying, as simple as it might be, there must be something written somewhere about this.

I am being serious.  Specific wording will likely vary a little by airline, but probably consist of a puffed up version of my first comment, i.e. "in the event of emergency evacuation, flight attendants shall assist those who are mobility impaired to the exits..."  So back to my question, are you looking for specific words like "place your hands under the armpits and lift upwards..."?  Again, not trying to be snarky, but just not sure what you are looking for in an answer here...

@MichaelHall yeah stuff like that, for example @ A.I. Breveleri's point was a good one, he/she can't be the first nor the last one.

Please take a look at my edit and let me know if you think it is an improvement.  I will delete my comments if you agree...

Actually, I think this is a dupe with a good answer already:  https://aviation.stackexchange.com/questions/42947/how-are-disabled-passengers-evacuated-during-emergency?rq=1

@MichaelHall yeah, that was great, the pdf has a lot of info, it even talks about a blind person traveling with their dog., etc...


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Why does this curved wind tunnel rotate?
Why does this  curved wind tunnel rotate, why is it not just curved?
A curved wind tunnel has a radial pressure gradient, but this pressure gradient doesn't exist in a real turn, so how do you get zero pressure gradient in a tunnel section?

It would seem rotation is required to get the outside of the car model to go faster than the inside.  One might also consider turning the model on a track with all motion generated by the model, as in real life.

@RobertDiGiovanni Flow in curved tunnel is already faster outside then inside...

Ah, but is it, especially when the apparatus is turning in the opposite direction (viscosity) of flow.  I would imagine flow velocity and turn rate would have to be fine tuned to achieve desired results.  The design has merit.

This isn't an Aviation question, and is off-topic here. The video is about an automotive wind tunnel for studying car designs, and has no aviation application.

OK, we'll take the car model out and put a little airplane model in there, OK?  Isn't understanding airflow related to aviation?

The figure looks like it is trying to illustrate a curved duct with physical walls. Is that correct? Can you edit the question to provide a source for it?

@RobertDiGiovanni -- re "Ah, but is it, especially when the apparatus is turning in the opposite direction (viscosity) of flow."  -- bingo.  As seen by outside observer, airflow speed in tunnel is actually zero, at least ideally, wall to wall, at any given instantaneous location of test model.  But same observer sees model travelling faster at outboard wingtip than at inboard wingtip.

(ctd)  Observer onboard test model, on the other hand, sees difference in airspeed from one wingtip to other, just as they would if tunnel were not there at all (discounting the disturbance that would arise from repeatedly flying through own wake.)  The whole idea becomes more ingenious the more it has time to "sink in" what is really going on here--

While superficially similar, this curved windtunnel solves a very different problem from a straight windtunnel. A straight windtunnel is used to study steady flight without needing a very long straight line. A rotating windtunnel is used to shield from wake turbulence from the last pass through the circle.
To simulate straight flight, you could just fly an airplane in a straight line. However, for any meaningful test duration, you would need a very long and straight section of air, and all your instrumentation would need to travel with the plane. Hence, a windtunnel moves air instead of the plane, which is cheaper than a sled on a very long rails.
To simulate curved flight, you could just mount an airplane on a rotating boom, like a merry-go-round, going in circles in stationary air. Unlike straight flight, you 're-use' the same bit of air by just going in circles. However, there is one problem: once you've gone full circle, you enter your own wake, which is not stationary and may contain turbulence. The curved windtunnel from your link is nothing more than a very fancy 'wind shield' that makes sure the rotating plane flies through exactly stationary air (stationary relative to an outside observer), without any wake from the previous pass.
A stationary curved windtunnel would not be the same, because now the air mass needs to curve. This introduces very different phenomena. Perhaps easiest to understand is the pressure gradient, because the air is 'flung' to the outside when going in a circle by centrifugal forces. On top of that, the flow path is disturbed because of Coriolis forces, which are less intuitive but just as real.

So when tunnel rotate radial pressure gradient is zero? How  coriolis forces appear in  tunnel?

When the tunnel rotates, the air is stationary. The radial pressure gradient is then zero, and there are no coriolis forces. Because from the perspective of the air, it's just sitting still, so there are no forces.

If tunnel dont rotate, why coriolis force exist if we dont have air movement in radial direction, air particles moves in pure circular path?

@user707264 That is correct, however: when you put a scale model in the airflow, the air particles no longer go in an exactly circular path. So then they do experience a coriolis force, and the resulting flow field is different.

But why coriolis force exist if tunnel dont rotate?  example:If earth dont rotate then coriolis force dont exist for wind..

@quietflyer Yes that is correct, if I sit on rotating merry go around, coriolis force is zero but if I try to walk  toward center or outward then is not zero..

@user707264 -- re "But why coriolis force exist if tunnel dont rotate?" - the tunnel rotating is actually not what eliminates the coriolis force. The stationary air inside the tunnel, as seen by an outside observer, is what eliminates the coriolis force. As the present answer implies, the tunnel rotating is just a way for the tunnel walls to stay out of the way of the test model as the test model moves in a circle through stationary air. See my (revised) answer for another (theoretical) way to move a test model around in a circle through stationary air, that wouldn't involve a rotating tunnel.

@user707264 -- but we could also look at it another way-- take a normal wind tunnel, bend into a curved shape, and you have a way to start to study flow in turning flight.  But you have problems due to pressure gradients and coriolis effects inside the rotating airmass.  Spin the whole thing in the opposite direction, and those effects are eliminated.  That's a way to look at it that doesn't focus on the fact that the chunk of air being approached by the test model at any given instant is actually stationary as seen by an outside observer.

Here you'll find his paper about the wind tunnel. Apparently the rotation has been introduced in order to compensate for the Coriolis effect.

Key point from another related answer:

A stationary curved windtunnel would not be the same, because now the
air mass needs to curve. This introduces very different phenomena.
Perhaps easiest to understand is the pressure gradient, because the
air is 'flung' to the outside when going in a circle by centrifugal
forces. On top of that, the flow path is disturbed because of Coriolis
forces, which are less intuitive but just as real.

So we keep the air stationary, and we move the test model around in a circle.
(See 2:04 in the video to see the model moving in a circle.)
As seen by an outside observer, the air inside the tunnel should actually not be moving at all.
More specifically, this should be true of the air at the location where the test vehicle would normally be, at any given instant, as seen by an outside observer, in a test run where no test vehicle is actually present.
So the fan should not pump air through the tunnel at high speed.  The fan should only provide enough force to prevent the test model, and the ducting itself, from pulling the air inside the tunnel along with it as the whole apparatus whirls around in a circle.
What does this really mean?  What is meant by the statement at 1:42 in the video where the narrator states that "the axial fan is required to produce the required pressure overrides to overcome the losses within the system"?
The three-dimensional geometry involved in the airflow inside the rotating ductwork is complex.  What does it really mean to say that the airflow approaching the moving vehicle at any given instant has zero velocity as seen by an outside, stationary observer?
From the point of view of someone in the test vehicle, we can imagine that we have a tube coming down from above that is magically constantly "unbending" somewhere out in front of us to continually provide a new segment of horizontal (curving) tube full of air (air that is in motion relative to us, but stationary relative to the outside world) for us to pass through.  So air that was formerly above us, is constantly being brought down to our level.  We pass rapidly through that airmass, which is then released out of the open end of the tunnel behind us.  Since the tunnel walls move along with the test model, there is clearly always a non-zero airflow velocity relative to any given point on the wall of the wind tunnel.  Looking at the apparatus as a whole, air is constantly being pumped in through the top and exiting at the bottom.  Yet the instantaneous velocity of a parcel of air as it is approached by the moving test vehicle is always zero as seen by an outside observer.  Admittedly, this is a bit of a mind-bender!
Here's yet another take on the whole problem: A rotating ductwork apparatus of the shape described in the video is one way to take the airflow from a fan and decelerate it to zero velocity, as seen from the viewpoint of an external, stationary observer.
Could we accomplish the same thing while leaving the walls stationary?

Initial response: No, because if the model is simply spinning around inside a donut-shaped chamber, you have no way for the fan to exert a force on the air inside that chamber in any particular direction to keep the air inside the chamber from being dragged along with the moving vehicle.  For the fan to do any good, it has to be inside a duct with a beginning and an end, one portion of which is bent into an arc of the appropriate curvature.  If you try to spin a model inside something like that, you'll soon hit a wall.  The only way to spin a test vehicle around inside something like that, is to rotate the tunnel along with the test vehicle.

But, actually... Possibly.  The tunnel could be an enclosed donut (torus) with stationary walls, with the test model whirling around inside, supported by an arm protruding through a slot in the inner wall of the tunnel, or possibly attached to a little motorized or magnetically-propelled cart that runs around the tunnel on a track like a toy train.  On the opposite side of the tunnel, supported in the same fashion and moving around the tunnel at the same speed, we could have a fan that blows just hard enough to decelerate the air in the tunnel to zero windspeed as seen by an outside observer.  The purpose of this fan is to prevent the test model, supporting arms, and any other moving apparatus inside the tunnel from eventually dragging all the air in the tunnel along with it.  Straightening vanes behind the fan could smooth out any vortices or other turbulence and ensure that the air downwind of the fan was truly stationary as seen by an outside observer.  One significant advantage of this arrangement would be that since the tunnel walls are stationary relative to the air in the tunnel, especially the air immediately in front of the moving test model at any given instant, we wouldn't have wall friction to contend with in our quest to create a uniform of chunk of air with zero windspeed (as seen by an outside observer) immediately in front of the moving test model. But we do have the complication of devising a method to drive the test model (and the fan) around inside the enclosed tunnel, or if we are mounting them on whirling arms with a central pivot point, we'll need a slot in the inner wall of the tunnel for the arms to protrude through.

Some other key points from the video:

In a turn, the airflow is different than when travelling linearly.

In a turn, the airflow is curved, as seen from the vehicle's perspective.

This means that the flow is faster over the "outside" side of the vehicle than over the "inside" side of the vehicle.

This also means that the angle of the (free-stream) airflow approaching the front of the vehicle is different than the angle of the (free-stream) airflow leaving the rear of the vehicle.

As has been explored in other ASE questions and answers, these effects are extremely relevant to airplane flight dynamics as well.  For example, at least in the special case where rate of curvature of flight path, yaw rate, and pitch rate are all constant, the "aerodynamic damping" effect on pitch rate and yaw rate can be also viewed as a change in local angle-of-attack (e.g. of the horizontal stabilizer, vertical stabilizer, etc) due to the free-stream relative wind curving in accordance with the curvature of the flight path.
Some other links relating to the curvature of the free-stream "relative wind" in turning (circling) flight:
Other ASE questions or answers:
What is airflow direction in turn? (Answer) -- see especially the related links at end
How is a "turning polar" constructed? (Answer)
What has happened to make me experience negative G with the control stick FULL AFT near the top of a loop? (Answer)
External links:
Section 6.1.10 "Long-Tail Pitch Effect" from John S. Denker's "See How It Flies" website
Section 8.10 "Long-Tail Slip" from same website
"Spiral Stability and the Bowl Effect" series from "Model Aviation" magazine, by Blaine Beron-Rawdon-- pertains to stability and control of rudder-controlled rc sailplanes-- the "bowl" refers to the curving lines of the free-stream relative wind in circling flight

Part 1
Part 2

@Sanchises -- On further reflection, I now agree completely w/ the other related answer https://aviation.stackexchange.com/questions/104426/why-does-this-curved-wind-tunnel-rotate/104439#104439 .   I just didn't fully see it, till I finished typing this answer!

I'm using "free-stream airflow", "free-stream relative wind", and "relative wind" interchangably here.  To mean the instantaneous airflow direction that would be seen at any given point on the vehicle, if the physical presence of the vehicle were not having any influence on the airflow direction.  The curvature I'm referencing is entirely due to the the fact that the trajectory of the vehicle is curved.  So the vehicle is rotating as well as linearly translating, and at any given instant two randomly chosen points on the vehicle are likely not moving through the air in the same direction.

So the focus is on the direction of the undisturbed airflow, nothing to do with eddies, turbulence, etc.  Could possibly explain that a little more clearly and/or use more consistent terminology throughout the answer.

"Could we accomplish the same thing while leaving the (TUNNEL) walls stationary?
Initial response: No, because if the model is simply spinning around inside a (STATIONARY) donut-shaped chamber,"

An automobile with front steering pivots on its rear wheels.  This may cause a significant amount of side lift in front from a change in the relative wind, making it more difficult for the tires to hold the turn.
A aircraft may have some side lift at the nose in a turn, but this is controlled by rudder and elevator (wing).  In a properly coordinated turn, the effects will be minimal.  However, in uncoordinated flight, effects may also be significant.

flow curvature is achieved in the absence of a pressure gradient

This is why the apparatus rotates.  The outside wall turns away from the flow and the inside wall turns towards it.  This (ingeniously) means the airflow from the diffuser frame of reference travels in more of a straight line (minimizing pressure gradient), whereas from the test model perspective it is curved. (This model is actually traveling in a circular path through straight air flow!).
Awareness of lift created by a change in relative wind (by certain shapes) helps us understand how a rounded fuselage can actually promote spin tendencies in aircraft once they have "gone flat", having the relative wind passing over the fuselage at a proper angle to generate a lifting force in the spin direction.

RE "an automobile with front steering pivots on its rear wheels." -- actually I'd suggest that for any given position of the steering wheel, the car tends to adopt a position where front and rear wheels are all "cutting" across the curving line of the actual trajectory at equal and opposite angles, i.e. the curving line of the instantaneous trajectory is striking the outside walls of the front wheels and thus creating a nose-to-inside torque, and also is striking the inside walls of the rear wheels and thus creating a rear-to-outside/nose-to-inside torque.

Said torques are expended to overcome any aerodynamic "damping" or aerodynamic resistance to yaw rotation. Also to supply torque to increase yaw rotation rate.  If aerodynamic damping in yaw is minimal, and yaw rotation rate is constant rather than increasing, so zero net torque is required, then all wheels can essentially be fully in line with the curving arc of the instantaneous trajectory, regardless of which wheels are actually being "steered" by the steering wheel. Above theory is for CG approx midway between front and rear wheels, may need to be slightly modified if this is not the case.


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Why are the guns on the AC-130 not mounted on the underside of the aircraft?
Why must the guns of the AC-130 be on the side forcing the plane to circle the target. Would it not be more practical to have the guns underneath the plane shooting downwards?

By putting the guns on the side, the gunner can direct a continuous stream of bullets at the intended target for as long as the pilot wishes to continue circling it. If the guns were belly-mounted, the pilot would have to make multiple passes at the target zone from different directions, with long pauses between them- long enough for the enemy soldiers to reposition themselves and escape.

Additionally: If the enemy were in a large circle, the pilot could fly a tight circle to aim belly-mounted guns. With side-mounted guns, the pilot can fly whatever size circuits are necessary, from tight to wide, allowing the gunners the luxury of aiming wherever the fire is needed.

Not to mention it's kind of hard to land if all your gun barrels are pointing out the bottom. If the pilot also circles the target at the center, it makes the target essentially static for the gunner whereas a belly mounted gun would be basically be a flyby shooting and difficult to aim.

This is not a full explanation: just because the gun is belly mounted does not mean the pilot has to make multiple passes. The pilot can still make circles around the target, the guns are not limited to shooting straight down. See @slebetman’s for some extra explanation

@ThomasW - In a Turn on a Pylon (Or Pylon Turn), the pitch angle will constantly vary with groundspeed which varies with wind speed and direction. You are keeping a constant throttle/power setting. And, your bank angle should be constant and determined by your distance or proximity to the pylon or center of your circle. This causes the aircraft to keep a constant distance from the pylon. The bank angle can be as little as 10° for a large circle, or as large as 60° for a tight circle. At times, to keep belly-mounted guns on target would require angles of inclination greater than 90°.

@ThomasW - This would require a belly mounted turret with plenty of clearance away from the bottom of the aircraft. This clearance would require a redesign of the landing gear to provide ground clearance when not airborne. The C-130 was designed for field expedient loading and unloading with minimal mechanical intervention. You can walk or push something up its low, shallow angle tail ramp with relative ease compared to an aircraft with a floor 6 feet off of the ground. The AC-130 was not a total redesign. It was a refit of an already existing platform.

@ThomasW OP said "have the guns underneath the plane shooting downwards"

Would belly-mounted guns be better than side-mounted guns to defend against aircraft attacking from beneath, for example as used on the B-17? And so, if the AC-130 was not expected to be in that situation, it would not be provisioned with belly-mounted guns?

The C130 is a cargo aircraft. The AC130 is the airframe converted for use as a gunship, of which there are several variants.
Guns under an aircraft are usually found on helicopters, mounted on turrets. Purpose built aircraft usually integrate guns into the wings (fighters) or fuselage (A10). WW2 bombers like the Flying Fortress worked more like the AC130, and had gunners in various places including a belly mounted turret.
The low load floor of a cargo aircraft precludes placement of guns there. Additionally, the AC130 carries multiple guns. 7.62 Minigun, 25mm Gatling gun, 40mm Bofors cannon, and 105mm howitzer have all been used. Simultaneous use requires multiple mounting points. Same side placement means they can all be used on one target.

105mm Howitzer in operation.

"Purpose built aircraft usually integrate guns into the […] fuselage (A10)" – I think the Warthog is more the other way around :-D The entire aircraft is built around the gun.

@JörgWMittag - The A-10 is the just the means to get the gun to the fight.  It has the side benefit of protecting the heck out of the gunner. The old saying for mounted ground troops is speed equals security. Combine that with the equivalent of a flying APC, you have a winning combination. If you have ever heard the GAU-8 go off, it is like the fart of God. It will blow away anything it touches from almost any range. Deadly, but not silent.

Minor point, the 7.62 miniguns haven't been on any gunship in well over 20 years. The H model gunships got rid of their 20mm cannons in the 1990's, and the 7.62's were out of service well before that.

I can only imagine that a 105mm Howitzer going off in an aircraft registers as an EVENT. Most pilots of other craft would probably just assume that something major had just broken and look to set down ASAP.

The original concept of the AC-130 gunship was to loiter over an area for an extended period of time in support of ground troops. It provides close air support when precision is needed instead of indiscriminate strafing runs of fast movers like F-4s and other jets. Using a ground reference maneuver called turns on pylons and pivotal altitude keeps one side of the gunship perpetually aligned with a target on the ground as the aircraft circles it.
You can find more detail on the Smithsonian Channel show Air Warriors.

I am quite sure that a strafing run, with a fighter, but more usually an attack aircraft like A-1 (Sky Raider), A-6 (Intruder) or A-10 (Warthog), is more precise than sideways fire from a gunship. Instead the advantage of a gunship is that it can maintain fire for minutes or even hours where a strafing run lasts just a second or two.

@JanHudec  The gunship has a sophisticated fire control computer integrated with a HUD aiming system. It is extremely precise. Also, the low relative target speed provides time to make small adjustments in aim to avoid collateral damage. The downside to the gunship is it's limited to night ops as it's a big slow target in daylight. The strafing aircraft are much more survivable in daylight, but as you indicate the runs are very short.

@JanHudec - Considering the computer aided fire controls, they are both very accurate. An attack fast mover like the A-1, 6, and especially 10 are very advantageous against a larger, not very agile, target like a vehicle. Whereas the AC-130 has the advantage over the aforementioned aircraft when engaging enemy personnel in close proximity to friendly personnel. It also has the advantage of being able to move its primary guns in relation to the aircraft to engage a target. The others have to move the entire aircraft. Other than a helicopter, it is the epitome of close air support of troops.

@JanHudec - Plus, the AC-130 and it’s predecessor, the AC-47, flew during the Vietnam War. The dedicated close air support A-10 did not fly until after the war ended. There was nothing like it in the sky at the time for that role.

seems reasonable precise: https://www.youtube.com/watch?v=zbRZwooTYtk gun camera footage starts at 1:33

Because the AC-130 is an airplane, not a helicopter, the only way to "hover" over a target is to fly in a circle around the target. This is called "loitering". This is the primary use-case for the AC-130: it's basically a flying artillery position, not a flying tank.
Having guns point downwards will have the guns point away from the target (due to banking) when loitering. Making it unsuitable to be used as a flying artillery position and forcing it to behave like a bomber (flying, reusable "bullet/shell"*).

"Having guns point downwards will have the guns point away from the target (due to banking) when loitering" Not if they're mounted on a belly-mounted turret, akin to an upside-down tank turret.

@nick012000 True, but when the OP said "guns underneath the plane shooting downwards" that pretty much indicates what he has in mind is guns pointing straight down, more or less.

@nick012000 It would be pretty silly to go to all the bother of engineering upside-down guns on the bottom of the aircraft and then, as soon as you go into a banked orbit, point then side-ways at maximum extent. Much simpler to point them out the side to begin with.

I think you've got this the wrong way round - you don't fly in circles because the guns are on the side; the guns are on the side because you have to fly in circles.
The only way to get a fixed-wing aircraft to loiter over a particular spot is for it to fly in a circle. To do this you have to bank the aircraft (i.e., tilt it on its side). If you do this, you automatically present the side of the aircraft to the target, so it's the natural place to position your guns. If you put them on the belly, you'd have to then aim them to the side - as far as they would go.
This side-firing strategy grew out of a technique developed by the British Chindits in Burma in WW2. They used small fixed-wing aircraft for supply and evacuation. The planes could not land in the jungle, so flew tight orbits over a clearing and lowered supplies on a rope. They then hauled up casualties the same way. In the Vietnam War, the Americans had the idea of replacing the rope with a line of fire and the gunship was born.


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Why does the P-175 polecat have two separate air intakes instead of 1 like most other UAVs?
I understand that the P-175 polecat has two engines which normally would mean one air intake for each engine, however other aircraft have used 1 air intake structure for two separate engines such as the Eurofighter Typhoon and the B-2. Furthermore it seems that most flying wing UAVs these days tend to have one centreline air inlet. Wouldn't it have saved some complexity to have both engines fed by a single air intake?

I see two inlets for both the B-2 and the Typhoon.

The problem with questions like this is that the answer is likely to be classified, or company confidential. That said, aviation is about compromises. The designers will balance competing needs for aerodynamics, engine efficiency, structure and other considerations and a design will emerge. Some designs will have two intakes, others just one, depending on how these competing needs are met.

@Jim, to be fair the typhoon's inlets to my knowledge are connected together and the B-2 has 2 inlets but 4 engines whereby each inlet feeds 2 engines each. My question perhaps should be phrased as inlet opening/port.

As other answers have indicated, there are a lot of complex tradeoffs involved -- the details of which may be classified and/or proprietary.  Those who know the answer can't answer -- the rest of us can just guess.
When designing an aircraft for low observability (in this case radar), you need to shield the fan blades from visibility from outside the aircraft.  Consequently, LO aircraft end up burying the engines and using S-duct or covered inlets.
For a single-engine aircraft, this may place the inlet on the center line like an X-47B.  In order to hide an inlet, it seems you need an S-duct with an offset of about one engine diameter.  For a centerline installation, this means your aircraft thickness at the center line is at least two engine diameters.
Turning the flow a lot in a short distance can cause separation from the duct walls.  A large amount of turning makes for a long duct -- more duct losses -- more weight -- longer aircraft, etc.  Nobody wants a long duct.
By splitting the inlet in two and laying each S-duct on its side, the offset may be reduced to one engine radius (each).  This may make for a shorter duct.  The maximum overall thickness may be kept to one engine diameter.
This also opens up some centerline real estate on the aircraft.  Something can now go between those inlets.  It may not be important in this case, but sometimes locating satellite dish, cameras, other sensors, lasers, radar, accelerometers, gyros, etc. on the centerline can have advantages.  At a minimum, weight and balance prefers a symmetrical aircraft so anything that you only have one of is most easily placed on the center line.  Famously, the A-10 valued a centerline location of the gun so much that the nose gear is offset.

Air inlets design is normally a straightforward matter.
It has to provide the engine(s) with a clean, sufficient and relative slow airflow. In particular, for a standard turbofan or turbojet, airflow speed should be reduced to some Mach 0.4/0.5 which is the optimal value for the engine's first-stage compressor. And as a rule of thumb, for each 1% of pressure lost in the inlet, the engine loses some 1.5% thrust. That's why an inlet is basically a pipe with a gently increasing section which, according to Bernoulli, slows the airspeed down and increases pressure.
Furthermore, inlets are normally a separate structure sticking out of the fuselage therefore adding additional weight and drag to the aircraft. For these reasons, inlets should be as few, short, straight and compact as possible.
Their positioning depends on the airplane's mission. For an aircraft like the B-2, the F-117 and the P-175, engines are mounted inside the wing both to reduce radar and infrared traceability and to have a free fuselage for the payload. For the same reason of reducing traceability, inlets and outlets are mounted above the wing. Having one engine (or one couple of engines) on each side of the wing, one single straight and short intake for each side of the wing is therefore the best solution in these cases.
On the contrary, having for example the RQ-170 Sentinel one single engine, it is better served with only one intake over the nose.


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Is a fighter pilot allowed to fly two different model fighters?
Does a fighter pilot who flies the F/A-18E have permission to fly the F/A-18F for training a new pilot? Or does an F/A-18E pilot fly with an F/A-18F for night bombardment?

This would very likely depend on the specific country/military branch. Are you wondering about any specific countries in particular?

Typically yes, but they generally won't be training a new pilot one day and flying a night bombing sortie the next.
Two seat trainers are intentionally very similar to their single seat counterparts.  (for obvious reasons!)  A pilot training in a squadron to replace front line fleet F/A-18E pilots will likely conduct early stage familiarization flights in the F model, but later solos will be in the E model.  And once training is complete the operational squadron he/she is assigned to will only fly the E model.
After an operational tour, if a pilot is assigned to an instructor tour there might be a refamiliarization flight in the F model as part of the IUT (Instructor Under Training) syllabus, plus a written test on any procedural or limitations differences.  Since the front seat of the two seat trainer is in the same location as the seat in the single seat model, the student will sit in front for most flights, with the instructor in back.  The IUT syllabus would emphasize the change in perspective when flying formation, and performing or observing other maneuvers while sitting in the rear seat.
In summary, a pilot would be cleared to fly either model as part of the training process for being designated as a qualified instructor.
They then might fly the F model for early stage flights, and the E model as formation lead for flights later in the syllabus.  But they wouldn't go on a night bombing mission as a shore-based instructor, unless of course it was a training flight.

"But they wouldn't go on a night bombing mission as a shore-based instructor", or it was war time and there was desperate need for pilots...  (Just saw some shows on what the Nazis did during WWII as they ran out of pilots.)

Sure, and this happened during Kosovo when I was an instructor.  But those instructors who volunteered to go were temporarily assigned to an operational squadron.  Generally training and combat are kept separate.

It should be mentioned that an instructor has to learn to fly from the rear seat, which is much bigger difference than between the front seat and the single-seat variant.

@Jan Hudec, Good point, I made an edit.  Please let me know if you think this could be improved further.  Thanks.

I would suspect that a pilot qualified on a Super Hornet could move between the two air frames just fine. Near as I know they both go through the transition course at NAS Lenore.  But aside from the Blue Angels I don’t know of any strike fighter squadron that uses both -E and the -F Super Hornets simultaneously


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How does the wing planform shape help to operate the aircraft from subsonic to supersonic speeds?
Consider a compound delta wing aircraft with design cruise Mach number of 2:  Explain how the wing planform shape helps to operate the aircraft from subsonic to supersonic speeds

I'm so sorry that people downvoted your question. It's a great question about the supersonic flight, and you're a new contributor too.

This wing formation helps in supersonic flight because it had reduced wing surface, creating less drag. But it still allows for the lift needed at those speeds. It the concorde had normal wings, once they reached about supersonic speed, there would be so much stress put on the wings, that it would break off. There are many other reasons, for example wingtip vorteses, and reduced drag, but the main one is because of stress.
Welcome to aviation stack exchange, hope you have a great time here! (Psst... im a new contributor too.)

Don't want to discourage you, but over time you'll start to notice that when ppl post obvious student assignment questions because they can't be bothered doing the research the question is intended to require, we try to encourage them to do the homework and post only when they're stumped. Just something to keep in mind.

thanks very greatful for you

@John K: Yeah, true, but what does homework mean? Because I don't really know what you mean by homework. I you could explain that, that would make my day.

@AirCanada001 if the question reads like something copied and pasted from a textbook exercise, as in "Explain how", that's a pretty good sign.  After a while on here it'll start to become obvious to you.

@John K: Thank's for explaining. I'll make sure I try to follow those guidelines.


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How do aircraft on the deck of a moving aircraft carrier align their INS/IRS?
I know that the F/A-18C is equiped with a rotary switch for inertial alignment. The switch has two positions, one for ground and the other for carrier operations. Leaving aside the GPS/GNSS, I wonder how is the alignment process in a moving vessel. I'm thinking in the old good days of Phantoms, A-4's and all other Navy aircrafts of the fifties and sixties where GPS was not yet implemented. How are vessel movement errors corrected?

Beyond a certain point it's just plain hard.  PBS did a documentary called "Carrier" where, among other things, they followed one exercise mission on the high seas and a half dozen planes ended up stuck in the sky for hours trying to land on the pitching deck.  They kept sending up refuelling craft to keep the fighters airborne and the pilots made attempt after attempt, failing over and over until they finally all managed to land.

@J... Were the refueling craft able to land easier than the other planes?  Or were they sourced at long range bases rather than from the carrier?

@MichaelRichardson Refueling craft have much greater endurance and range  than fighters.  They can land later or elsewhere.  I don't recall all of the details of this particular incident- you can watch the show if you like.

@MichaelRichardson More reading on it available here, under Night of the Pitching Decks.

The aircraft carrier has its own inertial navigation system, (SINS - Ship's Inertial Navigation System) and this information is transmitted to each individual aircraft INS by either cable or datalink.
From the EA-6B NATOPS flight manual of the Mid-1990s, pre-GPS era:

20.2.1.5  Carrier Alignment requires ship's movement information as well as present position.  This information must be supplied by either
SINS or manual entry.  When a
carrier alignment is called for, the system checks that the data link
is on.  If this is true it is assumed that an automatic alignment is
intended.  If the data link is off or in test a manual alignment is
assumed.  The CDI then provides cues for
the type of intended alignment that is assumed.

From there it goes into more detailed procedures.  There is no information about preferred use of UHF data link vs cable other than a mention that on the CDI (Control Display Indicator) there will be an indication of either "CV UHF" (data link) or "CBL" if alignment information is being received via cable.
Most times at man-up the plane captains will have the alignment started, or the front seat NFO would start the alignment during preflight.  Use of either UHF Data link or cable depended mainly parking location and number of available cables at that location, and speed/reliability of the UHF signal.  Generally the split was random, and 50/50.
CV Manual alignments were least common, least desired, and only performed when SINS information was not available.  They required more work making manual inputs, (of which ship's heading would need to updated if the ship was turning into the wind for launch, which it always did!) took longer, and they were less accurate.

Do you have a source for this? I'm wondering when it was implemented.

I just added  amplifying info from my flight manual of the 1990s.  Are you asking when the INS was implemented, or when GPS was common enough to obsolete it?

@Michael Hall, Great answer and many thanks for editing :)

I was USAF not NAVY, but I remember being surprised when I first saw the cockpit of a Navy F-4, and there was no INS in it.  On reflection, of course it made sense, but this was early 1970s through the late 1980s. So the boats probably did not have INS until sometime during that period.

Aircraft carriers are ships, not boats.

@rclocher3, you are of course technically correct, but to Naval Aviators they are boats.  Only the Surface Warfare Officers concern themselves with such distinctions.

@rclocher3 As former Marine C-130 aircrew: what's the difference?

@drmuelr I'm a former sailor who was stationed on an aircraft carrier, and a boat is a vessel small enough to be hoisted onto a ship.


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Is rudder deflection sufficient to maintain directional control on the runway at high speed, or is nose wheel steering needed?
If a fighter jet veers off the runaway, is nosewheel steering needed to counter, or can directional control be maintained with the rudder deflection alone?  Is there a chance that the rudder deflection alone won't be enough to change the aircraft direction?  Is there a speed zone or gap were the rudder deflection is not aerodynamically available, while at the same time steering of the front landing gear is ineffective due to loss of friction?  Or does the max speed for nosewheel steering and minimum speed for rudder effectiveness overlap each other?

Are you asking of rudder authority vs directional wheel authority during taxi, takeoff or landing rolls?

Exactly ,and if dead zones are between this two.

you could include relevant terms in the question (rudder authority, steering wheel authority, take off roll, taxi, and/or landing roll).

I just made some pretty major edits to capture what I think you are asking, because I think it is a good question, (as I interpreted it...) and doesn't deserve to be closed.  George, I realize that English is your second language, but you should really try to get some help with your basic grammar before making a public post on an English language site.  It can be very difficult sometimes to figure out what you are asking.

There is some overlap in effectiveness, and therefore no real magic number, but most aircraft that don't have full time nosewheel steering will have some published airspeed at which one will be favored over the other.  For example, in the EA-6B it was 80 KIAS.  On take off roll you would hold the nosewheel steering button on the control stick down to maintain directional control with the nose wheel.  Accellerating through 80 KIAS it was a call-out to release the NWS button and rely on the rudder effectiveness to keep control up to rotation at around 145 KIAS.  The reason for this was not because the nosewheel was less effective due to less friction, (traction) but rather there was a risk of overcontrolling steering at higher speeds.

Thanks for your time , now if the crosswind  comes in to play have the rudder deflection the power to overcome that ,or the differential brake control the aircraft ?

There's no single correct answer because it depends on the airplane and amount of crosswind, but I have never had to use differential braking because neither NWS nor rudder were enough.

That's a good idea for me to work with.

@Michael Hall: While my experience is limited to single-engine propellor planes, I'd have to wonder if that's really even an answerable question.  I mean I don't actually think about what I'm doing, I just do it.  Like riding a bike...

@jamesqf, yes, with full time NWS it really doesn't matter - you simply react with your feet appropriately while deflecting both rudder and nosewheel.  You really don't know or care which on is having the greater effect.  However, in a jet the sensitivity needed to turn sharply at slow taxi speeds is way to much for the much faster take off and landing speeds.  For this reason, the switch is generally springloaded to the disengaged position and must be held down to engage steering.  So I think the question is answerable.  As I interpreted it...

Yes, the steerable nosewheel and the rudder always work together.  They both contribute a yaw torque in the same direction.  As to your question about a speed regime where neither is effective-- I can't really answer that from first-hand knowledge across the entire aircraft design spectrum but it seems that the answer must be "no" in most cases or else the aircraft in question would not survive for long.


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Do helicopters use Ground speed or True airspeed?
Is there a limit on the speed and minimum altitude of the helicopter before switching between the two?

This question is a little bit unclear.  Are you asking if helicopters used true airspeed or ground speed?

Why do you think helicopters would be different from other aircraft?

I can't even tell how the text lines up with the title question.

Could this question have to do with hovering?  If you are landing and taking off with wind present then your transition to weight on wheels will take place at zero ground speed. In the hover you are also maintaining zero ground speed.  Once in forward flight indicated airspeed is used for normal flying.

Helicopters, like any other aircraft, can make use of true airspeed read outs as well as groundspeed readouts.  There are no altitude or speed limits for using the two. Helicopters do, depending on the air frame, have a limits as to a maximum forward air speed due to retreating blade stall problems.  This limit would be in calibrated airspeed, as opposed to a true airspeed or ground speed.

It depends on how you define the term “use”, but most aircraft, helicopters included, use indicated airspeed, most of the time.
Indicated airspeed is used because the airspeed indicator indicates indicated airspeed.  And the airspeed indicator is a primary flight instrument.
TAS and GS are useful references in flight planning and navigation, but they aren’t used to fly the airplane.  Most small GA aircraft don’t display TAS, and GS would only be displayed on a GPS navigation screen.  Any “switching” between them would not be a function of airspeed, but instead a function of your purpose and need.
For example, if you want to run a quick time/distance calculation to a point you would use ground speed.  (Actually if you have ground speed displayed in flight your system probably gives you time to go). TAS is mostly useful for determining zero wind ground speed in preflight navigation planning.
I am less familiar with helicopters, but I would surmise that options to use either TAS or GS as a reference would be similarly driven more by their instrumentation package than the means of producing lift.
It’s important to understand the distinctions, but inflight your life depends on indicated airspeed.  That’s what the dial or EFIS tape shows, and that’s what pilots fly by 99% of the time.


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What material is used for the skin of a modern fighter jet?
Modern passenger jets/airliners use aluminum 2 to 3 mm thick as their skin, their maximum speed is 900 km/h give or take. Modern fighter jets travel at twice that speed on average.
What material is used to make their skin. If it's aluminum, is it thicker and why? Is it because of the extra cruising speed?
If specifics are required, I ask about the F-35.

"passenger fighter jets" ?

Passenger Civilian aircraft

The F35's skin consists of a/several composite(s). Although it's obviously classified, I suspect the main reason it's chosen has something to do with stealth rather than strength. Interestingly, there seem to be more information about the F22 than F35, which is supposed to be, err, more secretive? For example, the F22's airframe bulkheads and twin booms are known to be constructed from titanium, and there are aluminum elements in its wings, and its skin is composite as well. That's pretty much all that's let on.
If you are asking for some not-so-secretive fighters, they are made from more mundane materials, not that these are any less expensive:

"Modern fighter jet" and "F-35" are not synonymous. The F-35 is just one out of ~20 fighter jets currently in production, and it's not representative of the rest of them.
The majority of fighter jets have their skin made out of aluminum, usually 7000 series. Thickness is 1.5-3 mm, a bit thinner than passenger airliners have. Fighters rely on their internal longerons rather than skin for global strength, so the skin is lightly loaded and not structurally essential. It then doesn't matter much that aluminum's strength falls greatly at elevated temperatures.
There is also less force put on the airframe at high speeds than it can handle. Fighters are only able to maneuver well at 200-400 knots, below the speed of sound. On average, they travel at Mach 0.7-0.8, similar to passenger jets or slower, as they're not as aerodynamically clean. Supersonic flight is generally only used for interception, in a straight line, for short periods of time, to chase another aircraft.
The F-35 is different. Its skin is made out of CFRP throughout. The matrix is CYCOM 977 epoxy resin in most fuselage sections, CYCOM 5250 bismaleimide resin in the wings and some hotter surfaces. The reinforcement is intermediate modulus carbon fiber, mostly Hexcel IM7, with 550-700 ksi tensile strength. Bismaleimides generally offer better heat resistance than aluminum; epoxies are a bit less or as heat resistant as aluminum, depending on the specific alloys and resins being compared.
That said, the F-35 has a lower top speed than most other modern fighters. Aluminum has proven to handle the higher heat stress in faster fighters just fine. But CFRP is a bit lighter, less prone to permanent bending than metal, and provides much better adhesion with the outer layer of resin-embedded RAM. The latter two properties make composites essential for VLO (all-aspect stealth) aircraft.
P.S. More on various resins: What are typical resins used in carbon fiber for aircraft? Are any resins impossible to recycle?


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What is in 'stealth paint' to absorb radar, and how does it work?
What is in the 'stealth paint' that coats stealth bombers e.g B-2 spirit, and stealth aircraft, e.g SR-71, and how does it absorb/deflect radar?

Reflecting radar is exactly what you don't want stealth paint to do...

Oops, I thought it reflected it!

Stealth paint doesn't reflect radar, it absorbs it by converting it to heat which is absorbed and dissipated by the airplane's structure. Paint is only one of several Radar Absorbing Materials available.
The paints known have suspended balls of ferrous material which absorb radar energy at certain frequencies and convert the radio energy into mechanical vibration, which causes friction and in turn heat.

Reflecting is fine, as long as it scatters the reflection.

Formulae for stealth paint seem to be closely guarded secrets. In electromagnetic test chambers we use radiation-absorbent materials (RAM) to absorb stray fields. I have seen stealth paints offered to motorists wishing to avoid police radar speed traps (some hope there!). They appeared to be ferromagnetic powder mixed with some kind of binder (back in WWII the Horten brothers experimented with carbon soot in glue). Certainly, ferromagnetic materials tend to be less bulky than other RAM solutions and their operational bandwidth can be optimised by tailoring the formula in various ways. Early stealth coatings were also reported in the press to wash off in the rain and the planes were repainted for every mission, which gives some clue as to the difficulty of finding a durable binder which is also electromagnetically transparent.

The physics fundamental is what happens when an incident electromagnetic waves changes mediums of different speed-of-light transmission index.  In the case of the transition is very low energy microwaves going from air medium to metal medium which is millions orders of magnitude slower speed of light results in total reflection. Radar absorbent paints work by using a high refractive index (to microwaves) coating to try to exploit destructive interference but that success depends on many factors like angle of incidence, wavelength and specific coating thickness. Radar absorbent materials depend on the specific spectral absorbance to convert wave energy into heat energy (like water getting hot when it absorbs 2.4 Ghz microwaves).  But again the success depends on the coating material and the frequency of the radar.  The biggest factor is geometry.  You can consider an aircraft with a shiny mirror surface and a radar as a flashlight beam.  If you can make the plane with faceted surfaces the mirrors don't reflect the flashlight beam in a consistent manner, only intermittently.  There is no such thing as true stealth, only "Low Observability". This is all knowledge one can gain from college level chemistry and physics degrees.

"millions orders of magnitude". Physics rarely deals with more than a dozen orders of magnitude, astrophysics perhaps with 3 dozen. In reality, this effect is about 1 order of magnitude, so that part of the answer is off by millions of orders of magnitude. Destructive interference only works if the coherence length is sufficient, and you can't rely on the enemy to cooperate here.

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